For reducing nitric oxide emissions of combustion engines it is known to arrange an SCR-catalytic converter in an exhaust system of the combustion engine, in which a selective catalytic reduction can be carried out. By apportioning a suitable reduction agent upstream of the SCR-catalytic converter, the nitric oxide can be reduced to nitrogen and carbon dioxide. A suitable reduction agent in this case is a watery urea solution. Through thermolisys and hydrolysis the urea can be transformed into ammonia, which on the SCR-catalytic converter makes possible an effective conversion of the nitric oxides into nitrogen and carbon dioxide.
Problematic on introducing the watery urea solution is the fact that in the hot exhaust gas the water component of the injected mixture evaporates more rapidly than the urea component, so that it can happen that concentrated urea accumulates on comparatively cold walls of the exhaust system, where it crystallises. During the crystallisation, by products such as biuret and melamine can be created, which no longer can be broken down so easily or not at all. Such residues lead to a contamination of the exhaust system and can impair its operation. Furthermore, these deposits reduce the effectiveness of the exhaust gas cleaning, since a part of the urea is not available for forming ammonia.
The present invention deals with the problem of stating an improved embodiment for an exhaust system of the type mentioned at the outset, which is characterized in particular in that the risk of a crystallisation of the urea is reduced.
With the present invention, this problem is solved in particular through the subjects of the independent claims. Advantageous embodiments are the subject of the dependent claims.